This invention relates to demodulation of a known optical signal, and more particularly relates to a simple and inexpensive demodulation of a digital optical signal whose characteristic of perceptible, yet distinctly consistent, square-signal shape deterioration (dispersion) is indicative of transmission in an optical fiber.
In the past, simple threshold detection has accomplished signal discrimination with the advantage of low cost, but with several disadvantages, including:
(1) the fact that short and medium lengths of fiber cause a dramatic change in the signal amplitude requiring large adjustment of the threshold amplitude level of the threshold detectors. Some manufacturers have taken to selling a standard length of fiber optic cable for a transmitter receiver pari with a set threshold, regardless of the length the customer might want at the time or at a later time. Imposing additional fiber length restrictions on the customer often destroys the cost advantage of the simple threshold detector approach.
(2) increased error rate when used with long lengths of fiber,
(3) incapability of discriminating between "noise", that is, spurious signals, having sufficient amplitude to turn the threshold detector on, and appropriate signals.
In addition to simple threshold detection, "sequenced voting devices" have been used. In this approach, a digital signal is broken up into a number of slices, 16 for example, which are each "voted on" by single level or multilevel threshold detectors. If a certain number of threshold detectors affirm the presence of a digital signal, then a fixed digital signal is regenerated. The disadvantage of the sequenced voting devices has been their expense, and they are highly data-speed sensitive.
It is noted that the Harris Corporation is currently marketing a Model 4359, "fiber optics digital data link," and that Valtec Corporation is currently marketing a "fiberoptic duplex data link--long distance, model RSK-D1" and a "fiberoptic duplex data link--moderate distance, model RSH-D1".